This one is about the 'parachute effect' inside the car. When cruising at a speed of 80-85 kmph there is enough atmospheric friction that any moving object would encounter. All the fighter planes have a needle/nose in front to negate the atmospheric friction they encounter at Mach speeds and above. Car manufacturers design the car after studying the 'air-flow' around the shape of the car in a computer stimulated environment. When car makers study it, they just take it for granted that windows of the car would be closed. On a highway, if you just keep any one of the four windows open or just the 2 windows at the rear or in front open, there is inlet for air, but there is no outlet which results in a 'parachute effect' inside the car. 'Parachute effect' puts more pressure on the engine where in it consumes more fuel to provide the same thrust. At the same time if you keep one of the front window and one of the rear window open it facilitates cross ventilation and reduces the drag on the engine.
It is easy for me to go on and on about it with many examples...but I want to keep the blog short. For more on drag coefficient click http://en.wikipedia.org/wiki/Drag_coefficient
In any case, best thing to do is to keep all 4 windows closed when on a highway for better energy efficiency.
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